Cement is a hydraulically hardening construction material which consists of a mixture of finely ground, non-metallic inorganic constituents. In general, it is produced by jointly grinding the burnt cement clinker with other major and minor constituents.
The main raw material for clinker production is limestone which is mined in quarries, pre-comminuted in crushers and conveyed into the cement plant. After grinding and drying, it is mixed with other ground components such as sand, clay or iron ore to obtain a raw meal. This raw meal is burnt to clinker in a rotary kiln at temperatures above 1450° C. and then cooled in a cooler to a temperature of below 200° C. Subsequently, the granules obtained are ground to cement in a ball mill together with gypsum or anhydrite (cf. “Integrated Pollution Prevention and Control (IPPC)”, Reference Document on Best Available Techniques in the Cement and Lime Manufacturing Industries, European Commission, Brussels, 2011; Gasafi, E., Jeske, U. and T. Reinhardt, 2006, “Gipsreduktion mit Kohlenstoff-Rahmenbedingungen für die Verwertung mineralischer Reststoffe mit Sulfat und potentielle Einsatzstoffe für ein GRC-Verfahren”, publication series “Wissenschaftliche Berichte Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft” FZKA-7189, Karlsruhe).
According to a study of the World Business Council for Sustainable Development, the cement industry is responsible for about 5% of the global anthropogenic CO2 emissions (cf. “The Cement CO2 Protocol: CO2 Emissions Monitoring and Reporting, Protocol for the Cement Industry, Working Group Cement of the World Business Council for Sustainable Development (WGC-WBCSD), Oct. 19, 2001, http://www.wbcadcement.org/pdf/co2-protocol.pdf). Since about half of the CO2 emissions during the clinker production are caused by the raw material limestone, the reduction of the clinker content (clinker factor) by replacement of another component can provide a substantial contribution to the reduction of these emissions.
As cement substitute, calcined clay was proposed for example. The calcination of fine-grained mineral solids, such as clay, conventionally is effected in rotary kilns or multiple-hearth roasters. The maintenance of a low temperature at a retention time necessary for the treatment with this method is ensured thereby. The U.S. Pat. No. 4,948,362 for example describes a method for calcining clay, in which kaolin clay is treated in a multiple-hearth roaster by means of a hot calcining gas to increase gloss and minimize abrasiveness. In an electrostatic precipitator, the calcined clay powder is separated from the waste gas of the calcining furnace and processed further, in order to obtain the desired product.
From DE 10 2008 031 165 A1 it is known to use the plant for producing the cement itself for the production of calcined clay, wherein at least two preheating lines are provided, of which one serves for preheating the clay and the other serves for heating clinker raw material. In a combustion chamber hot gases are produced, which serve the calcination of the clay and are guided through the preheating stages in counterflow to the solids.
The clay used in these processes, however, has a high kaolin content of more than 40 wt-% and is very expensive, so that no economically marketable clinker substitute can be produced therefrom.